1. Field of the Invention
This invention relates generally to position sensors incorporating Hall-effect devices and more particularly to systems for tracking the position of EGR valve plungers.
2. Description of the Related Art
Exhaust gas recirculation (EGR) valves are used to improve the performance of automobile engines by reducing the temperature within the cylinders during combustion and by preventing pre-ignition. At high temperatures, nitrous oxides (NOx), a pollutant, tend to form. Also at high temperatures, the fuel air mixture within the cylinder tends to pre-ignite in the absence of a spark during the compression stroke, rather than during the combustion stroke.
To reduce the temperature within the cylinder, the EGR valve permits small amounts of inert gas from the exhaust system to enter the cylinder during the intake stroke. The EGR valve typically has a plunger whose position determines the amount of exhaust gas permitted to reenter the cylinder. In some systems the plunger is coupled to a diaphragm that is controlled by vacuum from the intake manifold. In others, the plunger is controlled electronically by a solenoid or stepper motor. Pneumatic or hydraulic control may also be used.
The EGR valve typically remains closed while the engine is cold or at idle and remains open once the engine has warmed up and is running at part-throttle. Where the EGR valve fails to open properly, NOx emissions increase and pre-ignition may result. If the EGR fails to close properly, rough idling, hesitation, and stalling may occur.
In many systems, an emissions control module (ECM) or other onboard computer monitors the EGR valve to ensure proper functioning. In systems with solenoid or stepper motor driven EGR valves, the ECM or onboard computer controls the position of the plunger of the EGR valve.
It is important that the position of the plunger be measured with precision in order to provide accurate information to the ECM of a vehicle. Some positions sensors use a Hall-effect device which measures changes in the magnetic field incident thereon. In some systems, the Hall-effect device moves relative to a stationary permanent magnet or magnets to vary the magnetic field incident on the Hall-effect device with position. In others, one or more permanent magnets move relative to a stationary Hall-effect device. In either case, in such systems, the device involves two distinct parts that move independently.
Such systems are inconvenient to service. The Hall-effect device and permanent magnet are both affected by the harsh environment of an automobile engine compartment. The Hall-effect device is typically formed in silicon chips which are prone to failure at high temperatures. A typical iron based ferromagnetic material will lose its permanent magnetism at approximately 1400° F., which is the temperature of the exhaust gases of an internal combustion engine under heavy operating conditions.
Inasmuch as either of these parts may fail, they ought to be readily accessible for replacement. However, separate mounting of the permanent magnet and Hall-effect device requires the replacement of two parts. Furthermore, it is difficult to precisely position the permanent magnet with respect to the Hall-effect device inasmuch as they are mounted to distinct structures, one of which is movable. Accordingly, replacement of either the magnet or the Hall-effect device requires recalibration of the position sensor.
It would therefore be an advancement in the art to provide an EGR position sensor integrating the permanent magnet and Hall-effect device into a single unit and providing for convenient servicing and calibration.